Foundations, Footers, and Slabs

Conventional foundations, footers, and slabs use a lot of concrete, which is energy-intensive and polluting to produce—up to a ton of CO2 is released in producing a ton of cement. Admixture components such as calcium chloride (an accelerator), gypsum (a retarder), and sulfonated melamine formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen." (SMF, a plasticizer) also affect the environmental impact of the concrete. Depending on the chemical, the impact may be on-site or at the plant. Also, concrete foundations and slabs do not provide much by way of thermal insulation, though they can provide thermal storage.

Preferred techniques and materials

Several factors come into play when builders strive for a green approach to foundation, footer, and slab installations.

Foundations, footers, and slabs should always be detailed to reduce thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel.
as much as possible.

Forming can account for a significant portion of the total cost of poured concrete. Essentially, things get built twice: once in forms and again in concrete. Plywood has been the mainstay of concrete-forming companies for many years, though some companies have invested in reusable, durable forms—a more resource-efficient solution that is still relatively labor-intensive. Reusable forms also require form-release agents, most of which are petrochemical-based and off-gas large amounts of volatile organic compounds (VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.). Vegetable-based form-release oils are available.

For large buildings and some sites with poor soil conditions, foundations are engineered to specific structural requirements and fairly extensive use of concrete may well be necessary. For certain applications, replacing up to 50 percent of the cement in a concrete mixture with fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required.
Expanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) foam should be preferred until extruded polysterene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) foam insulation becomes available without ozone-depleting HCFCs (hydrochlorofluorocarbons, a class of chemicals that have been used to replace chlorofluorocarbons, or CFCs, which cause even more ozone depletion than HCFCs). Some EPS foam products have an integral borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments. treatment, which helps keep damaging insects out of the foam. The brominated flame retardants used in most EPS foam have health and environmental risks that are generating significant concern.

Precast concrete foundation walls are available in some areas. They use less concrete than site-cast foundations and are designed to accommodate interior insulation.

The depth of a foundation wall (and thus the amount of material required) can be reduced by raising the frost line, generally by placing foam insulation horizontally (usually extending about 4 feet) around the foundation. These “frost-protected shallow foundations” can save money and materials where crawl-space or slab-on-grade foundations are used in cold climates. “Rubble trench” foundations are another option—one that was favored by Frank Lloyd Wright. Pier foundations, which can reduce excavation requirements and concrete use significantly, may also be an appropriate choice.

Non-asphalt-based dampproofing reduces the risk of introducing chemicals into local aquifers and VOCs into the building, and they can last longer.

Other alternative products

In some parts of the country, rigid mineral wool panels are available that help insulate foundation walls while also providing effective drainage. Mineral wool typically includes iron-ore slag—a pre-consumer waste product. And recycled aggregate or crushed glass can be used as aggregate in the concrete or as backfill for foundation drains.

Foundations can also be designed with termite shields or be backfilled with special termite-proof sand, so that toxic soil-treatments are not required. Pesticides commonly used around foundations introduce hazardous chemicals to the environment and must be periodically reapplied.

Conventional foundations, footers, and slabs use a lot of concrete, which is energy-intensive and polluting to produce—up to a ton of CO2 is released in producing a ton of cement. Admixture components such as calcium chloride (an accelerator), gypsum (a retarder), and sulfonated melamine formaldehyde (SMF, a plasticizer) also affect the environmental impact of the concrete. Depending on the chemical, the impact may be on-site or at the plant. Also, concrete foundations and slabs do not provide much by way of thermal insulation, though they can provide thermal storage.

Our guide lists products from the Greenspec® Online Directory of environmentally preferable building products. Manufacturers and/or their representatives should visit this page to learn more about suggesting their products for inclusion.

I just came across this post, and am curious about what you ended up doing. I wondered why the same footer you used on the exterior walls (maybe a tad larger) would not suffice for the interior load bearing wall. Thanks. john

1.
Sep 5, 2009 10:11 PM ET

slab on grade foundations by ron

I am building a slab on grade 24ft. X 24ft. cottage in upstate pa. I will be using a 12"X6" footer with 5 course of 8" block around the parimeter foundation. On top of that i will have a 4" slab. the frost depth is 36" I will have a interior load bearing wall running through the center of the structure.What is the approved method of support, under that interior load bearing wall and pad.